1. Field of the Invention
The present invention relates to a detector for detecting whether or not a passenger sits in a normal posture on a seat having a seat cushion supporting the passenger thereon and a seat back supporting the back of the passenger, that is, whether or not a passenger sits at a suitable position on the seat cushion with his or her back resting at a suitable position on the seat back. In a non-limitative manner, the present invention relates to a passenger sitting state detector for monitoring the presence of a passenger sitting in a normal posture on a vehicular seat for the purpose of preventing the passenger from hitting an object by means of an air bag in the case of a collision of a vehicle.
2. Description of the Related Art
An air bag device which is designed for protection of a passenger in a vehicle and which deploys an air bag in case of a collision of the vehicle has been in practical use. However, when there is a passenger sitting in a certain posture or state, even such an air bag device may not be able to provide effective protection. For example, in an air bag device which is designed for front collision and which deploys an air bag between a passenger and an instrument panel to protect the passenger, if the air bag is deployed when the passenger is located at a position close to the instrument panel such as a front end of the seat, it is impossible to ensure a sufficient space for deployment of the air bag and a sufficient length of time for contact of the passenger with the air bag. As a result, the air bag device cannot provide effective protection.
In recent years, it is not rare to see an air bag device installed in a front seat (a passenger seat) next to a driver seat. Once the air bag is deployed, it cannot be reused with ease. Hence, especially in the case of such an air bag, it is preferable to impose restrictions on the operation of the air bag so that it is deployed only when there is a passenger sitting on the passenger seat.
For example, Japanese Patent Application Laid-Open No. HEI 7-285864 discloses a device which detects the presence of a passenger sitting at an unsuitable position for deployment of an air bag, such as a front end of a seat, by means of pressure sensors disposed in front and rear portions of a seat cushion and which issues a warning to the passenger.
Although Japanese Published Patent No. HEI 7-89150 and Japanese Patent Application Laid-Open No. HEI 7-270541 do not particularly relate to an air bag, these publications disclose a device for detecting the presence of a passenger by means of a sensor which detects changes in dielectric constant of a dielectric material installed in a seat cushion. In a short-range sensor for monitoring changes in capacitance, a short-range detection electrode (ATs) changes in capacitance due to environmental changes around a sensor portion (soaking in water, changes in humidity and the like). Especially in the case where the short-range sensor is installed in a vehicular seat, changes in humidity tend to cause changes in capacitance and thus bring about a failure of detection. In the short-range sensor disclosed in Japanese Published Patent No. HEI 7-89150, the facts that environmental changes such as changes in humidity cause gradual changes in capacitance and that changes in distance between a person and the sensor cause abrupt changes in capacitance are taken into account. That is, an electric signal corresponding to a capacitance is smoothened in a time-series manner to create a threshold value. If the electric signal exceeds the level of the threshold value, the sensor switches from a detection signal indicative of the presence of a person to a detection signal indicative of the absence of a person or vice versa. In Japanese Patent Application Laid-Open No. HEI 7-270541, a third electrode is added, and three capacitance values among the respective electrodes are calculated. Based on these capacitance values, only those changes in capacitance which are relevant to the posture of a passenger and unsusceptible to environmental changes are calculated.
Furthermore, Japanese Patent Application Laid-Open No. HEI 8-268220 discloses a device which detects the presence of a passenger or a close positional relation between the passenger and an instrument panel, and which changes an operating state of an air bag according to a result of the detection.
By the way, in recent years, apart from the air bag device designed for front collision, a side air bag device for protection of a passenger against a side collision has also been proposed and put into practice. In general, such a side air bag device is installed in a lateral portion of a seat back or a vehicular door.
FIG. 14 shows a child CHL sitting on a front seat (a passenger seat) next to a vehicular driver seat. In a vehicular seat SE shown in FIG. 14, a lateral portion of a seat back SEb on the side of a front-left door DfL has an opening SEso which is covered with a lid and which is designed for deployment of a side air bag. If an impact sensor integrated into a center pillar detects an impact in a direction perpendicular to the longitudinal axis of a vehicle to be sensed, for example, due to a collision of another vehicle with the door DfL, an ECU (an electronic control unit mainly composed of a CPU) installed in a center tunnel portion of the vehicle makes a judgment on the necessity to deploy the side air bag. If it is judged that the side air bag needs to be deployed, the ECU supplies a driver for the side air bag device in the seat back SEb with a signal indicative of activation. Then the driver supplies an activation circuit with electric current, thus activating the air bag. Thereby the side air bag is deployed and travels towards the front of the vehicle through the opening SEso in an expanding manner. However, when the child CHL leans against the door DfL as indicated by a solid line, even if the side air bag is deployed, it does not enter a space between the passenger (CHL) and the door DfL. Therefore, there is no need to deploy the side air bag. When the passenger (CHL) sits in a normal posture as indicated by an alternate long and two short dashes line, deployment of the side air bag is expected to produce a beneficial effect.
Whether the air bag device for front collision or the side air bag for side collision is used, deployment of the air bag at the time of a collision does not produce a sufficiently beneficial effect unless the passenger to be protected sits in a posture conceived by a designer of the air bag (hereinafter referred to as a normal posture, a correct posture or a predetermined posture). In the short-range sensor disclosed in Japanese Published Patent No. HEI 7-89150, only one detection electrode is provided in a seat cushion to simply detect whether or not there is a passenger sitting on a seat cushion. Thus, it is impossible to detect whether or not the passenger sits in a normal posture with respect to the seat back. According to Japanese Patent Application Laid-Open No. HEI 7-270541, a seat cushion and a seat back are provided with a first electrode and a third electrode respectively (an equipment ground portion such as a vehicle body serves as a second electrode) to detect three capacitance values. These capacitance values are used to calculate changes in capacitance caused by the passenger. Thus, it is again impossible to detect whether or not the passenger sits in a normal posture with respect to the seat back.